This application claims benefit under 35 USC xc2xa7 119(a) to Australian Provisional Patent Application Serial No. PR 5757, filed Jun. 18, 2001, which is incorporated herein by reference in its entirety.
This invention concerns an aircraft equipped for conducting airborne gravity surveys. In another aspect it concerns a process for creating airborne gravity surveys.
In airborne gravity surveys, and particularly in gravity gradient surveys the major contributor to measured signal is often the topography. In such situations, very careful modeling of the topographic gradient signal is required in order to correctly identify that residual part of the signal which constitutesxe2x80x94exclusive of other generally smaller deterministic disturbances such as self-gradientxe2x80x94the effect of target anomalies. Vital to this correction is access to a suitably accurate digital elevation map (DEM) which is properly registered to the aircraft position. Such a DEM must cover both the survey area and a sufficient boundary beyond the survey extent. However, it is the topography closest to the aircraft, which will have the most profound effect on the gravity gradient signal.
In a first aspect, the invention is an aircraft for conducting airborne gravity surveys, equipped with:
An inertial platform in which sits a gravity gradiometer, and which operates to provide attitude measurements.
A gravity gradiometer to provide the gradient of gravity.
A laser scanner having a range of at least 200 m, a scan angle of at least +/xe2x88x9230 degrees recorded to an accuracy of at least 0.5 degrees, and a scan rate of at least 10 scans per second with a laser pulse rate of at least 10 kHz, to return range and scan angle measurements from the ground.
A GPS antennae to receive GPS data from which timing and position data can be retrieved.
A processor to generate a digital elevation map (DEM) using the laser range, scan angle, aircraft attitude and aircraft position data, and from which the gradient of gravity of the topography can be calculated.
A second ground-based GPS antenna and receiver may be provided at a reference location for differential correction of the aircraft mounted GPS receiver position.
The ground return data obtained using the invention is across a sufficiently wide swathe so that very adequate DEMs over the whole survey area can be produced. As a result, the scanner DEM will be correctly registered relative to the aircraft, especially in the region close to the aircraft where topographic gravity gradient effects are not inconsiderable. Also, scanner DEM""s can be composed in remote regions where existing DEMs are inaccurate, out of date or unavailablexe2x80x94this enables the aircraft to collect valid data over almost any ground. Furthermore, the scanner DEMs will generally be more accurate than other commercially available DEMs.
A laser profilometer may be fitted adjacent to the scanner, to provide independent data to monitor the scanner integrity throughout a survey.
In order to transform scanner range data into ground return positions, it is necessary to combine the range data with a measure of the aircraft attitude, that is roll, pitch and heading available from the inertial platform, and the aircraft position available from the GPS. To do this the raw data streams from the laser scanner and the inertial platform are accurately time stamped with synchronisation pulses derived from the aircraft GPS. The raw GPS data from the aircraft and ground GPS receivers may be processed to provide sub-meter accuracy.
In another aspect the invention is a process for creating airborne gravity surveys using measured attitude data, laser range data and scan angle data, and aircraft position data. The process comprising the following steps:
Removing data having invalid values from the range data.
Interpolating the attitude and aircraft position data onto the range data time instances.
Vector rotating the range vector and if required the offset vector of the laser scanner from the GPS antenna about the GPS antenna to transform the range data into ground position data.
Discarding single point anomalies from individual selected scans.
Decimating the scans by selecting the points with the lowest ground position in a number of evenly spaced bins across the scan swathe.
Manufacturing a gridded version of the scanner ground position data set using the decimated ground return data.
Merging the gridded version with a less accurate but larger regional DEM.
The merging process may consist of the following steps: Overlaying the scanner DEM on a section of the regional DEM so that the regional DEM extends at least 5 km further than the scanner DEM in every direction. Tilting and shifting the regional DEM to match the scanner DEM at the boundary of the scanner DEM. And allowing the regional DEM to in-fill any internal gaps in the scanner DEM. The combined scanner and regional DEMs are used to calculate the gravity gradients which will result from the topography. The scanner DEM is used in the area it covers, while the regional DEM is used outside this area. There are public domain methods for this conversion of topography to gravity gradient.